A scanning image of a target object acquired with computed tomography (CT) technologies includes a plurality of pixels of different gray levels. Gray levels of the pixels are proportional to the density of the target to be scanned. The regions of different densities of the object to be scanned are of different X ray absorption levels. Thus, in a CT scanning image, the pixels of lower gray levels are indicative of low density regions of lower X ray absorption levels, and the pixels of higher gray levels are indicative of high density regions of higher X ray absorption levels. Generally, the pixels in a CT scanning image are represented with CT values (in Hu). For instance, the CT values of the pixels corresponding to objects or tissues of higher X ray absorption levels are set higher than the CT values of the pixels corresponding to objects or tissues of lower X ray absorption levels; for instance, the CT value of the pixels corresponding to a bone tissue is set as +1000 Hu, the CT value of the pixels corresponding to water is set as 0 Hu, and the CT value of the pixels corresponding to air is set as −1000 Hu, etc.
At present, the prior art has proposed a dual energy CT technology by which a target to be scanned is scanned with X rays of two kinds of energies and a dual energy CT scanning image of the target to be scanned is acquired. The information included in the dual energy CT scanning image is more sophisticated than the information included in the scanning image acquired by a single energy CT technology which conventionally uses X rays of only one kind of energy to scan the scanning target.
However, in the single energy CT scanning image and the dual energy CT scanning image, different objects in the target to be scanned may be of the same or similar X ray absorption levels. For instance, a bone tissue may be of the same or similar X ray absorption levels with a calcification portion of a blood vessel or other tissues caused by diseases, and thus they may have the same or similar CT values. Therefore, the objects indicated by pixels of the same or similar gray levels in a CT scanning image can hardly be distinguished from each other.
In addition, in order to highlight different objects in a target to be scanned, a contrast agent is applied (e.g. injected) to the target to be scanned before a CT scanning The CT scanning of an object to be scanned applied with a contrast agent is also called as CT contrast agent enhanced scanning, and the acquired CT scanned image is called as a CT contrast agent enhanced scanning image.
However, the CT value of the pixels corresponding to the contrast agent in a CT contrast agent enhanced scanning image is relatively high, so it is difficult to distinguish the pixels corresponding to the contrast agent from the pixels of calcification portions etc. which have a high density and a higher X ray absorption level in the CT contrast agent enhanced scanning image. In the prior art, there is a method for comparing a CT scanning image (plain scanning image) acquired by a CT scan (plain scan) before application of a contrast agent with a CT contrast agent enhanced scanning image (enhanced scanning image) to identify the pixels corresponding to the calcification portions. However, such a method requires two CT scans of the target, which increases the X ray absorbed dose of the target to be scanned. However, the CT value of the pixels corresponding to the contrast agent in a CT contrast agent enhanced scanning image is relatively high, so it is difficult to distinguish the pixels corresponding to the calcification portions from the pixels corresponding to the contrast agent and a bone tissue etc. which has a high density and a higher X ray absorption level in the CT contrast agent enhanced scanning image. In the prior art, there is a method for comparing a CT scanning image (plain scanning image) acquired by a CT scan (plain scan) before application of a contrast agent with a CT contrast agent enhanced scanning image (enhanced scanning image) to identify the pixels corresponding to the calcification portions. However, such a method requires two CT scans of the target, which increases the X ray absorbed dose of the target to be scanned.
Therefore, a method for identifying calcification portions in a CT contrast agent enhanced scanning image is desirable.